Electrolyte film and fuel cell

ABSTRACT

An electrolyte membrane includes inorganic oxide particles having proton conductivity, and an organic resin or an inorganic matrix component. The inorganic oxide particles having proton conductivity preferably comprise hydrated antimony oxide particles represented by the following formula (1) and have an average particle diameter of 5 to 50 nm, and the content of the hydrated antimony oxide particles is preferably in the range of 5 to 80% by weight in terms of an oxide (Sb 2 O 5 );
 
Sb 2 O 5 .nH 2 O  (1)
 
wherein n is 0.1 to 5. The electrolyte membrane provides a fuel cell which exhibits high cell performance even after prolonged operation and/or at high operating temperatures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrolyte membranes and fuel cellsusing the electrolyte membranes. More particularly, the inventionrelates to electrolyte membranes for fuel cells which can maintain ahigh voltage in a prolonged operation and/or an operation at hightemperature and have excellent stability, and fuel cells using theelectrolyte membranes.

2. Description of the Prior Art

As electric generating systems using clean hydrogen as the energysource, having high efficiency, causing no pollution and generating nowarming gas such as CO₂, fuel cells have recently received attention.Regarding such fuel cells, development research has been earnestlyundertaken for the purpose of using the fuel cells as fixed equipment inhomes or business places and as mobile equipment in automobiles.

Fuel cells are classified according to types of electrolyte membranesused therein, and they are divided into alkali electrolyte membranetype, solid polymer electrolyte membrane type, phosphoric acid type,molten carbonate type and solid electrolyte membrane type. In the solidpolymer electrolyte membrane type and the phosphoric acid type, thecharge exchange substance is a proton, so that the fuel cells of thesetypes are also referred to as “proton exchange membrane fuel cells”.

Examples of fuels used for the fuel cells include hydrocarbon fuels,such as natural gas, LP gas, city gas, alcohol, gasoline, kerosine andgas oil.

The above hydrocarbon fuel is first converted into a hydrogen gas and aCO gas by a reaction such as steam reforming or partial oxidation, andthe CO gas is removed to obtain a hydrogen gas. The hydrogen is fed toan anode and dissociates into protons (hydrogen ions) and electrons bythe function of a metal catalyst of the anode. The electrons flow to acathode through a circuit with doing work, while the protons (hydrogenions) diffuse into an electrolyte membrane and flow to a cathode. At thecathode, water is produced from the electrons, the hydrogen ions andoxygen fed to the cathode, and the water diffuses into the electrolytemembrane. That is to say, the fuel cells utilize a mechanism to drawelectric current during the production process of water from oxygen andhydrogen derived from a fuel gas.

Examples of the electrolyte membranes used for such fuel cells include acation-exchange membrane which is based on polystyrene and has sulfonicacid group, a mixed membrane of fluorocarbon sulfonic acid andpolyvinylidene fluoride, a membrane wherein trifluoroethylene is graftedon a fluorocarbon matrix, and a perfluorocarbon sulfonic acid membrane.

However, migration of protons through the electrolyte membranecomprising such an organic resin membrane, namely, ionic conductance ofthe membrane, depends upon a water content in the membrane. When aprolonged operation is carried out or a high-temperature operation at atemperature of about 80° C. or higher is carried out, the water contentin the membrane is lowered, and as a result, ionic conductance islowered to cause reduction of an output voltage.

On this account, Japanese Patent Laid-Open Publication No. 103983/1994proposes a solid polymer electrolyte membrane fuel cell in which acompound having a phosphoric acid group is contained in a polymermembrane to allow the polymer membrane to exhibit excellent waterretention properties and thereby which can be favorably employed at anoperating temperature of 80° C. or higher.

In Japanese Patent Laid-Open Publication No. 143723/2001, an electrolytemembrane comprising an amorphous silica molded product containingphosphorus pentoxide is disclosed as an electrolyte membrane for a fuelcell favorably employable at an operating temperature of 80° C. orhigher.

However, when the solid polymer electrolyte membranes thus proposed areused for a long period of time at a high temperature of 100° C. orhigher, the water content in the membrane is lowered because of hightemperature, or in case of an electrolyte membrane using a resin, protonconductivity is lowered by deterioration of the resin component tothereby reduce a voltage, resulting in a problem of lowering of cellperformance.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an electrolytemembrane capable of providing a fuel cell which can maintain a highvoltage even in a prolonged operation or an operation at hightemperature and has excellent stability. It is another object of thepresent invention to provide an electrolyte membrane capable ofproviding a fuel cell which exhibits high cell performance even when itis used in a prolonged operation or a high-temperature operation.

SUMMARY OF THE INVENTION

The present inventors have earnestly studied means to enhance cellperformance in the use of a fuel cell for a long period of time underthe high-temperature conditions. As a result, the present inventors havefound that antimony oxide particles have high proton conductivity andexhibit high water retention properties at high temperatures and that bythe use of these antimony oxide particles for an electrolyte membranetogether with an organic resin or an inorganic matrix component, a fuelcell which exhibits high cell performance even when it is used for along period of time and/or at a high temperature is obtained. Based onthe finding, the present invention has been accomplished.

An electrolyte membrane according to the present invention comprisesinorganic oxide particles having proton conductivity and a matrixcomponent.

The inorganic oxide particles having proton conductivity preferablycomprise hydrated antimony oxide particles represented by the followingformula (1) and have an average particle diameter of 5 to 50 nm, and thecontent of the hydrated antimony oxide particles is preferably in therange of 5 to 80% by weight in terms of an oxide (Sb₂O₅);Sb₂O₅.nH₂O  (1)wherein n is 0.1 to 5.

The matrix component preferably comprises an organic resin, and theorganic resin is particularly preferably at least one organic resinselected from the group consisting of a cation-exchange resin based onpolystyrene, a mixture of fluorocarbon sulfonic acid and polyvinylidenefluoride, a graft copolymer wherein trifluoroethylene is grafted on afluorocarbon matrix, a perfluorocarbon sulfonic acid resin, a vinylidenefluoride resin, a 2-dichloroethylene resin, a polyethylene resin, avinyl chloride resin, an ABS resin, an AS resin, a polycarbonate resin,a polyamide resin, a polyimide resin and a methacrylic resin.

The matrix component is preferably an inorganic matrix component, andthe inorganic matrix component particularly preferably comprises atleast one inorganic oxide selected from the group consisting of ZrO₂,SiO₂, TiO₂ and Al₂O₃.

The fuel cell according to the present invention uses the electrolytemembrane.

DETAILED DESCRIPTION OF THE INVENTION

The electrolyte membrane and the fuel cell according to the presentinvention are described in detail hereinafter.

<Electrolyte Membrane>

The electrolyte membrane of the invention comprises inorganic oxideparticles having proton conductivity and a matrix component.

<Inorganic Oxide Particles Having Proton Conductivity>

Examples of the inorganic oxide particles having proton conductivity foruse in the invention include antimony oxide particles, heteropolyacids,such as tungstic acid, stannic acid and molybdic acid, crystallinealuminosilicates into which rare earth ions have been incorporated, andporous crystalline aluminum phosphate.

The inorganic oxide particles need to have high water retentionproperties, and they are preferably hydrated antimony oxide particles.

The hydrated antimony oxide particles having proton conductivitypreferably used in the invention are particles of a hydrate of antimonyoxide and represented by the following formula (1) (except mere adhesivewater that is not crystallization water):Sb₂O₅.nH₂O  (1)wherein n is 0.1 to 5.

The antimony oxide particles have proton conductivity, and they areadded for the purpose of enhancing conductivity of the electrolytemembrane.

The hydrated antimony oxide particles have an average particle diameterof preferably 5 to 50 nm, more preferably 5 to 25 nm. If the averageparticle diameter is less than 5 nm, powder resistivity (volumeresistivity) sometimes exceeds 10¹⁰ Ω·cm. On this account, cationconductivity is low and a sufficient output voltage is not obtainedoccasionally. If the average particle diameter exceeds the upper limitof the above range, the hydrated antimony oxide particles cannot besufficiently introduced into the electrolyte membrane in some casesthough it depends upon the process for producing the electrolytemembrane, and even if they are introduced, strength of the resultingelectrolyte membrane sometimes becomes insufficient.

The water content in the hydrated antimony oxide particles, as measuredafter drying at 100° C. for 1 hour, is in the range of preferably about0.5 to 22% by weight, more preferably 2 to 22% by weight.

The water content in the hydrated antimony oxide particles, as measuredafter drying at 200° C., is in the range of preferably about 0.25 to 10%by weight, more preferably 0.5 to 10% by weight.

If the water content in the hydrated antimony oxide particles, asmeasured after drying at 200° C., is less than 0.25% by weight, aneffect of use of the hydrated antimony oxide particles as theproton-conductive inorganic particles is not obtained, and when ahigh-temperature operation and/or a prolonged operation is carried out,voltage tends to be reduced to deteriorate cell performance.

It is difficult to obtain hydrated antimony oxide particles having awater content, as measured after drying at 200° C., of more than 10% byweight.

The hydrated antimony oxide particles for use in the invention do notneed to have a water content in the above range during the preparationof the electrolyte membrane, and the water content may be adjusted to bein the above range by subjecting the electrolyte membrane to moisteningtreatment or the like after the electrolyte membrane has been prepared.

The hydrated antimony oxide particles for use in the invention have apowder resistivity (volume resistivity) of preferably less than 10¹⁰Ω·cm, more preferably less than 10⁷ Ω·cm.

If the volume resistivity of the conductive oxide particles exceeds theupper limit of the above range, an effect that the electrical resistanceis kept low becomes insufficient and a sufficient output voltage cannotbe obtained in some cases, though it depends upon the content of theparticles in the electrolyte membrane.

<Matrix Component>

The matrix component for use in the invention is an organic resin or aninorganic matrix component.

(Organic Resin)

The organic resin is not specifically restricted provided that it isemployable for an electrolyte membrane. For example, the organic resinis preferably at least one organic resin selected from the groupconsisting of a cation-exchange resin based on polystyrene having asulfonic acid group, a mixture of fluorocarbon sulfonic acid andpolyvinylidene fluoride, a graft copolymer wherein trifluoroethylene isgrafted on a fluorocarbon matrix, a perfluorocarbon sulfonic acid resin,a vinylidene fluoride resin, a 2-dichloroethylene resin, a polyethyleneresin, a vinyl chloride resin, an ABS resin, an AS resin, apolycarbonate resin, a polyamide resin, a polyimide resin and amethacrylic resin.

As such organic resins, resins exemplified in, for example, JapanesePatent Laid-Open publication No. 275301/1994, Japanese Patent Laid-OpenPublication No. 199559/1998, Japanese Patent Laid-Open Publication No.40737/1998 and Japanese Patent Laid-Open Publication No. 103983/1994 areemployable.

(Inorganic Matrix Component)

The inorganic matrix component is decomposed at high temperatures andpreferably comprises at least one inorganic oxide selected from thegroup consisting of ZrO₂, SiO₂, TiO₂ and Al₂O₃.

By the use of such an inorganic matrix component, an electrolytemembrane that is porous, excellent in membrane strength and waterretention properties, and excellent in heat stability and durability, isobtained.

As the inorganic matrix component, a metallic salt of Zr, Si, Ti or Aland/or a hydrolysis-polycondensation product of an organometalliccompound of Zr, Si, Ti or Al is preferable.

Examples of the metallic salts include chlorides, sulfates and nitrates.

Examples of the organometallic compounds include a silicic acidsolution, alkoxysilane, zirconium tetrabutoxide, silicon tetrapropoxide,titanium tetrapropoxide, and hydrolysates of these compounds.

The inorganic matrix component may be one formed from a conventionalsol, such as a ZrO₂ sol, a SiO₂ sol, a TiO₂ sol, an Al₂O₃ sol or aSiO₂.Al₂O₃ composite sol.

Of the above inorganic matrix components, a component comprising silicais preferable, and a component formed from a silicic acid solutionobtained by dealkalizing an alkali metal silicate aqueous solution or anorganosilicon compound represented by the following formula (2), such asalkoxysilane, is particularly preferable in the invention.R_(a)Si(OR′)_(4-a)  (2)

In the above formula, R is a vinyl group, an aryl group, an acrylicgroup, an alkyl group of 1 to 8 carbon atoms, a hydrogen atom or ahalogen atom, R′ is a vinyl group, an aryl group, an acrylic group, analkyl group of 1 to 8 carbon atoms, —C₂H₄OC_(n)H_(2n+1) (n=1 to 4) or ahydrogen atom, and a is an integer of 0 to 3.

Examples of such alkoxysilanes include tetramethoxysilane,tetraethoxysilane, tetraisopropoxysilane, tetrabutoxysilane,tetraoctylsilane, methyltrimethoxysilane, methyltriethoxysilane,ethyltriethoxysilane, methyltriisopropoxysilane, vinyltrimethoxysilane,phenyltrimethoxysilane and dimethyldimethoxysilane.

<Electrolyte Membrane>

The electrolyte membrane of the invention comprises the inorganic oxideparticles having proton conductivity, and the organic resin or theinorganic matrix component. The electrolyte membrane comprising theinorganic oxide particles having proton conductivity and the organicresin is also referred to as a “solid polymer electrolyte membrane”, andthe electrolyte membrane comprising the inorganic oxide particles havingproton conductivity and the inorganic matrix component is also referredto as an “inorganic electrolyte membrane”.

(Solid Polymer Electrolyte Membrane)

In the solid polymer electrolyte membrane, the inorganic oxide particleshaving proton conductivity are contained in the amount of preferably 5to 80% by weight, more preferably 10 to 50% by weight, in terms of anoxide.

When the content of the inorganic oxide particles having protonconductivity is in the above range, the solid polymer electrolytemembrane has high proton conductivity and has high water retentionproperties at high temperatures. Hence, a fuel cell that exhibits highcell performance even when it is used for a long period of time or at ahigh temperature can be obtained. If the inorganic oxide particleshaving proton conductivity are contained in an amount lower than thelower limit of the above range, an effect of use of the particlessometimes becomes insufficient. It is difficult to produce a solidpolymer electrolyte membrane containing the inorganic oxide particleshaving proton conductivity in an amount exceeding the upper limit of theabove range, and even if such a solid polymer electrolyte membrane canbe produced, strength of the membrane sometimes becomes insufficient.

The solid polymer electrolyte membrane of invention is obtained bysubstantially adhering (supporting) or introducing the proton-conductiveinorganic oxide particles into a membrane comprising the organic resin.On this account, the organic resin membrane is desirably porous anddesirably has a porosity of not less than 5%, preferably not less than10%.

(Process for Producing Solid Polymer Electrolyte Membrane)

There is no specific limitation on the process for producing the solidpolymer electrolyte membrane of the invention, provided that theinorganic oxide particles having proton conductivity can be adhered(supported) onto or introduced into the organic resin membrane. Forexample, the organic resin membrane is immersed in a dispersion of theinorganic oxide particles having proton conductivity to introduce theinorganic oxide particles into pores of the organic resin membrane andthen dried to obtain the solid polymer electrolyte membrane. Theoperations of immersion and drying may be repeated when needed, wherebythe amount of the proton-conductive inorganic oxide particles introducedcan be increased.

When a sol in which the inorganic oxide particles having protonconductivity are stably dispersed is used as the dispersion, a solidpolymer electrolyte membrane in which the inorganic oxide particleshaving proton conductivity are homogeneously dispersed can be obtained.When a mixed solvent of water and an alcohol is used as the dispersionmedium of the sol, affinity for the resin is increased, and therefore, asolid polymer electrolyte membrane in which the inorganic oxideparticles having proton conductivity are more homogeneously dispersedcan be obtained. As a result, even if a fuel cell using the electrolytemembrane is subjected to a high-temperature operation or a prolongedoperation, the degree of lowering of proton conductivity is low, and ahigh output voltage can be maintained.

After the organic resin membrane is immersed in the dispersion, themembrane is taken out, dried and then heated at a temperature in thevicinity of the softening point of the organic resin used, whereby theinorganic oxide particles having proton conductivity can be firmly fixedto the organic resin membrane. The inorganic oxide particles havingproton conductivity can be fixed also by sandwiching the solid polymerelectrolyte membrane between two electrode membranes after drying, andthen hot pressing them.

The solid polymer electrolyte membrane can be produced also bybeforehand dispersing the inorganic oxide particles having protonconductivity in a resin monomer and polymerizing the monomer. The solidpolymer electrolyte membrane of the invention can be produced also bydissolving the organic resin temporarily, mixing the resin with theproton-conductive inorganic oxide particles and molding the mixture intoa membrane by a conventional molding method.

In the present invention, the process comprising immersing the organicresin membrane in a dispersion of the inorganic oxide particles havingproton conductivity and drying the membrane is desirable from theviewpoints of strength of the resulting membrane and productionsimplicity.

(Inorganic Electrolyte Membrane)

In the inorganic electrolyte membrane, the inorganic oxide particleshaving proton conductivity are contained in the amount of preferably 5to 80% by weight, more preferably 30 to 75% by weight, in terms of anoxide.

If the content of the inorganic oxide particles having protonconductivity is lower than the lower limit of the above range,conductivity of the resulting membrane becomes insufficient to reduce anoutput voltage of a fuel cell, and an effect of use of theproton-conductive inorganic oxide particles having high water retentionproperties and conductivity cannot be sufficiently obtained in somecases. If the content of the inorganic oxide particles having protonconductivity is higher than the upper limit of the above range, theamount of the matrix component is small, and the strength of theresulting electrolyte membrane sometimes becomes insufficient.

To the inorganic electrolyte membrane of the invention, a componenthaving water retention properties can be added when needed. Examples ofthe components having water retention properties, which are preferablyemployable in the invention, include porous inorganic oxides, such assilica alumina, zeolite (crystalline aluminosilicate), clay minerals andtitanium nanotubes; and fine particles of inorganic compounds whereinthese porous inorganic oxides or other porous inorganic compounds havinghigh specific surface area are modified with sulfonic group, phosphoricacid group, carboxyl group or the like.

The amount of the component having water retention properties added ispreferably not more than about 30% by weight.

The membrane thickness of the inorganic electrolyte membrane of theinvention is in the range of preferably 0.01 to 10 mm, more preferably0.05 to 5 mm.

If the membrane thickness of the inorganic electrolyte membrane is lessthan 0.01 mm, sufficient membrane strength is not obtained, and when theresulting membrane is processed, cracking takes place or pinholes areproduced occasionally.

If the membrane thickness of the inorganic electrolyte membrane exceeds10 mm, proton conductivity is lowered, and when a membrane is formed bya coating solution method, sufficient membrane strength is not obtainedin some cases.

Since the inorganic electrolyte membrane comprises the inorganic matrixcomponent and the proton-conductive inorganic oxide particles havingexcellent high-temperature water retention properties and conductivityas described above, a fuel cell using the inorganic electrolyte membranecan stably maintain a high output voltage even if the fuel cell is usedin a prolonged operation and an operation at high temperature. That isto say, by the use of the inorganic electrolyte membrane, a fuel cellhaving high cell performance can be obtained.

The inorganic electrolyte membrane of the invention is usually porousand has a porosity of preferably not less than 5%, more preferably notless than 10%.

(Process for Producing Inorganic Electrolyte Membrane)

The inorganic electrolyte membrane of the invention can be produced bythe use of a coating material for forming inorganic electrolyte membraneobtained by dispersing a precursor of an inorganic matrix componentfunctioning as a binder and inorganic oxide particles having protonconductivity in a dispersion medium and if desired further dispersingfine particles of a component having water retention properties.

As the inorganic oxide particles having proton conductivity and the fineparticles of a component having water retention properties, thosepreviously described are employable.

Examples of the precursors of the inorganic matrix component include theaforesaid metallic salts and organometallic compounds of Zr, Si, Ti andAl. These metallic salts and organometallic compounds may have beenhydrolyzed or partially hydrolyzed. Further, a ZrO₂ sol, a SiO₂ sol, aTiO₂ sol, an Al₂O₃ sol or a SiO₂.Al₂O₃ composite sol may be used, aspreviously described. Furthermore, a mixture of the hydrolysate and thesol may be used.

The precursor of the inorganic matrix component is preferably a silicaprecursor. More specifically, a silicic acid solution obtained bydealkalizing an alkali metal silicate aqueous solution, a partialhydrolysate of an organosilicon compound represented by the aforesaidformula (2), such as alkoxysilane, or a hydrolysate thereof ispreferable. Examples of the alkoxysilanes include the aforesaid ones.

When at least one of the alkoxysilanes is hydrolyzed in, for example, awater/alcohol mixed solvent in the presence of an acid catalyst, adispersion of a precursor of a matrix component containing a hydrolysispolycondensation product of the alkoxysilane is obtained. Theconcentration of the precursor of the matrix component in the dispersionis in the range of preferably 1 to 15% by weight, more preferably 2 to10% by weight, in terms of an oxide.

The dispersion of the precursor of the inorganic matrix component, theinorganic oxide particles having proton conductivity, and if necessary,the fine particles of the component having water retention propertiesare mixed with a dispersion medium and dispersed therein to prepare acoating material for forming inorganic electrolyte membrane. The amountof each component mixed is determined so that the content of eachcomponent in the resulting inorganic electrolyte membrane should be inthe aforesaid range.

The total concentration of the precursor of the inorganic matrixcomponent, the inorganic oxide particles having proton conductivity, andif necessary, the fine particles of the component having water retentionproperties in the coating material for forming inorganic electrolytemembrane is in the range of preferably 2 to 50% by weight, morepreferably 5 to 30% by weight, in terms of an oxide.

If the total concentration of those components in the coating materialis lower than the lower limit of the above range, an inorganicelectrolyte membrane having a desired thickness cannot be obtained byone coating in some cases, though it depends upon the coating method. Ifthe total concentration of those components in the coating material ishigher than the upper limit of the above range, viscosity of theresulting material becomes high, and it is difficult to use it as acoating material. Even if it can be used as a coating material, thecoating method is restricted or the strength of the resulting inorganicelectrolyte membrane becomes insufficient in some cases.

As the dispersion medium, water is preferable, and an organic solventcan be used in combination. Examples of the organic solvents includealcohols, such as methanol, ethanol, propanol, butanol, diacetonealcohol, furfuryl alcohol, tetrahydrofurfuryl alcohol, ethylene glycoland hexylene glycol; esters, such as methyl acetate and ethyl acetate;ethers, such as diethyl ether, ethylene glycol monomethyl ether,ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,diethylene glycol monomethyl ether and diethylene glycol monoethylether; and ketones, such as acetone, methyl ethyl ketone, acetylacetoneand acetoacetic ester.

These solvents may be used singly, or may be used as a mixture of two ormore kinds.

In the present invention, the coating material may be used after it issubjected to deionization treatment or after it is concentrated or mixedwith a sol when needed.

There is no specific limitation on the process for forming the inorganicelectrolyte membrane using the coating material for forming inorganicelectrolyte membrane, provided that the resulting membrane can haveelectricity-generating properties and strength, and for example, therecan be mentioned a process comprising filling the coating material in amold of good release properties, such as a mold made of Teflon (R), andthen drying and heating it.

The inorganic electrolyte membrane can be formed also by a processcomprising applying the coating material for forming inorganicelectrolyte membrane onto an electrode of a fuel cell by spraying, rollcoating, flexographic printing or the like and then drying and heatingit. According to this process, an electrode and an inorganic electrolytemembrane can be produced integrally.

Further, the inorganic electrolyte membrane of the invention can beformed also by a process comprising beforehand applying a dispersion ofthe precursor of the inorganic matrix component to form a thin membranecomprising the inorganic matrix component, then spraying a dispersion,in which the inorganic oxide particles having proton conductivity and ifnecessary the fine particles of the component having water retentionproperties are mixed with a dispersion medium and dispersed therein,onto the thin membrane and then drying the membrane.

Fuel Cell

The fuel cell of the invention is characterized by using theabove-mentioned electrolyte membrane.

More specifically, the fuel cell comprises plural layers of unit cellslaminated one upon another through a cooling plate or the like. The unitcell comprises the electrolyte membrane and a pair of gas diffusionelectrodes (fuel electrode and oxidizing agent electrode) arranged onboth sides of the electrolyte membrane, and in the unit cell, theelectrolyte membrane is sandwiched between the fuel electrode and theoxidizing agent electrode, and on the outer sides of both theelectrodes, grooved collectors for forming a fuel chamber and anoxidizing agent chamber are arranged.

The gas diffusion electrode usually comprises a porous sheet in which acatalyst particle-supported conductive material is held by a hydrophobicresin binder such as PTFE. The gas diffusion electrode may be anelectrode in which a catalyst particle layer is provided on anelectrolyte membrane contact surface of the porous sheet comprising theconductive material and the hydrophobic resin binder such as PTFE.

The electrolyte membrane is sandwiched between a pair of the gasdiffusion electrodes, and they are contact bonded by a publicly knownmethod such as hot pressing.

The catalyst has only to be-one having a catalytic action on theoxidation reaction of hydrogen and the reduction reaction of oxygen andcan be selected from metals, such as platinum, ruthenium, iridium,rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt,nickel, manganese, vanadium, molybdenum, gallium and aluminum, andalloys of these metals.

The conductive material has only to be an electronic conductivematerial, and for example, carbon materials, such as publicly knowncarbon black, specifically furnace black, channel black and acetyleneblack; activated carbon; graphite and various metals; are employable.

Examples of the hydrophobic resin binders include various resinscontaining fluorine, such as polytetrafluoroethylene (PTFE), atetrafluoroethylene/perfluoroalkyl vinyl ether copolymer andperfluorosulfonic acid.

As the hydrophobic resin binder, a proton conductive polymer may beused. This polymer also has a function as a binder in itself and canform a matrix having sufficient stability of the catalyst particles andthe conductive particles in the catalyst layer.

On an opposite surface to the surface to be in contact with theelectrolyte membrane, a gas diffusion layer comprising the hydrophobicresin binder may be provided.

The amount of the catalyst supported may be in the range of preferably0.01 to 5 mg/cm², more preferably 0.1 to 1 mg/cm², when a sheet of thecatalyst layer is formed.

In order to obtain sufficient permeability, the electronic conductiveporous material preferably has a specific surface area of 100 to 2000m²/g. The average pore size of the gas diffusion electrode is preferablyin the range of 0.01 to 1 μm.

In the present invention, a proton conductive polymer layer may beformed on an interface between at least one of the catalyst layers andthe electrolyte membrane.

In the fuel cell of the invention, hydrogen is fed to a fuel chamber,while air (oxygen) is fed to an oxidizing agent chamber, and electricityis generated through the following electrode reactions.

Fuel electrode (anode): H₂→2H⁺+2e⁻

Oxygen electrode (cathode): 2H⁺+½O₂+2e⁻→2H₂O

It is considered that when hydrated antimony oxide particles are used asthe inorganic oxide particles having proton conductivity, hydrogen isbonded to oxygen of antimony oxide skeleton or is present in a state ofwater or is present in a state of proton (H⁺) or hydronium ion (H₃O⁺),for the antimony oxide particles in the electrolyte membrane.

Gaseous water or condensed water produced by the cell reaction rapidlypasses through the layer showing higher water repellency and having finepores in the oxygen electrode by virtue of the capillary phenomenon.

EXAMPLES

The present invention is further described with reference to thefollowing examples, but it should be construed that the invention is inno way limited to those examples.

Example 1

Antimony pentoxide particles (average particle diameter: 10 nm,Sb₂O₅.2.5H₂O) as inorganic oxide particles having proton conductivitywere dispersed in a mixed solvent (ethyl alcohol:water=50:50) to preparea dispersion having a Sb₂O₅ concentration of 30% by weight. In thisdispersion, a perfluorocarbon sulfonic acid membrane (A) (available fromDuPont, Nafion membrane N-117, thickness: 183 μm) as an organic resinmembrane was immersed at 50° C. for 12 hours. Then, the membrane wastaken out and dried at 100° C. for 12 hours to give a solid polymerelectrolyte membrane (A). The content of the antimony pentoxideparticles in the solid polymer electrolyte membrane (A), which wascalculated from an increase in weight, was 20% by weight.

Separately, to platinum-supported carbon particles having a platinumcontent of 40% by weight in terms of Pt, a mixed solvent of ethylalcohol and water (ethyl alcohol:water=50:50) was added to give a paste.Then, the paste was applied onto two sheets of carbon paper (availablefrom Toray Industries, Inc.), which had been subjected to waterrepellent treatment with tetrafluoroethylene, in such a manner that thedensity of the platinum-supported carbon particles on each sheet became0.5 mg/cm², and they were dried at 100° C. for 12 hours to prepare twogas diffusion electrodes (A).

The two gas diffusion electrodes (A) were used as a positive electrodeand a negative electrode. Between these electrodes, the solid polymerelectrolyte membrane (A) was sandwiched, and they were hot pressed at100° C. for 5 minutes under a pressure of 150 kg/cm² to prepare a unitcell (A) in which the gas diffusion electrodes (A) and the solid polymerelectrolyte membrane (A) were joined.

(Evaluation)

The unit cell (A) was subjected to moistening treatment at 80° C. and arelative humidity of 30% for 2 hours. Then, the unit cell (A) wasoperated at a current density of 0.5 A/cm² for 50 hours at atmosphericpressure and a temperature of 80° C., 100° C., 120° C. or 140° C., andan output voltage at each temperature was measured.

The results are set forth in Table 1.

Example 2

A solid polymer electrolyte membrane (A) obtained in the same manner asin Example 1 was re-immersed in the antimony pentoxide particledispersion at 50° C. for 12 hours, then taken out and dried at 100° C.for 12 hours to give a solid polymer electrolyte membrane (B). Thecontent of the antimony pentoxide particles in the solid polymerelectrolyte membrane (B), which was calculated from an increase inweight, was 35% by weight.

(Evaluation)

A unit cell (B) was prepared in the same manner as in Example 1, exceptthat the solid polymer electrolyte membrane (B) was used. Then, outputvoltages were measured.

The results are set forth in Table 1.

Example 3

A solid polymer electrolyte membrane (B) obtained in the same manner asin Example 2 was re-immersed in the antimony pentoxide particledispersion at 50° C. for 12 hours, then taken out and dried at 100° C.for 12 hours to give a solid polymer electrolyte membrane (C). Thecontent of the antimony pentoxide particles in the solid polymerelectrolyte membrane (C), which was calculated from an increase inweight, was 45% by weight.

(Evaluation)

A unit cell (C) was prepared in the same manner as in Example 1, exceptthat the solid polymer electrolyte membrane (C) was used. Then, outputvoltages were measured.

The results are set forth in Table 1.

Example 4

A solid polymer electrolyte membrane (D) was obtained in the same manneras in Example 1, except that antimony pentoxide particles (Sb₂O₅.2.5H₂O)having an average particle diameter of 40 nm were used. The content ofthe antimony pentoxide particles in the solid polymer electrolytemembrane (D), which was calculated from an increase in weight, was 15%by weight.

(Evaluation)

A unit cell (D) was prepared in the same manner as in Example 1, exceptthat the solid polymer electrolyte membrane (D) was used. Then, outputvoltages were measured.

The results are set forth in Table 1.

Example 5

A solid polymer electrolyte membrane (E) was obtained in the same manneras in Example 1, except that a perfluorocarbon sulfonic acid membrane(B) (available from DuPont, Nafion membrane N-115, thickness: 127 μm)was used as an organic resin membrane. The content of the antimonypentoxide particles in the solid polymer electrolyte membrane (E), whichwas calculated from an increase in weight, was 25% by weight.

(Evaluation)

A unit cell (E) was prepared in the same manner as in Example 1, exceptthat the solid polymer electrolyte membrane (E) was used. Then, outputvoltages were measured.

The results are set forth in Table 1.

Example 6

A solid polymer electrolyte membrane (F) was obtained in the same manneras in Example 1, except that a perfluorocarbon sulfonic acid membrane(C) (available from DuPont, Nafion membrane NE-1135, thickness: 51 μm)was used as an organic resin membrane. The content of the antimonypentoxide particles in the solid polymer electrolyte membrane (F), whichwas calculated from an increase in weight, was 15% by weight.

(Evaluation)

A unit cell (F) was prepared in the same manner as in Example 1, exceptthat the solid polymer electrolyte membrane (F) was used. Then, outputvoltages were measured.

The results are set forth in Table 1.

Comparative Example 1

A unit cell (G) was prepared in the same manner as in Example 1, exceptthat the perfluorocarbon sulfonic acid membrane (A) was used as a solidpolymer electrolyte membrane (G) without introducing antimony pentoxideparticles. Then, output voltages were measured.

The results are set forth in Table 1.

Comparative Example 2

A unit cell (H) was prepared in the same manner as in Example 1, exceptthat the perfluorocarbon sulfonic acid membrane (B) was used as a solidpolymer electrolyte membrane (H) without introducing antimony pentoxideparticles. Then, output voltages were measured.

The results are set forth in Table 1.

Comparative Example 3

A unit cell (I) was prepared in the same manner as in Example 1, exceptthat the perfluorocarbon sulfonic acid membrane (C) was used as a solidpolymer electrolyte membrane (I) without introducing antimony pentoxideparticles. Then, output voltages were measured.

The results are set forth in Table 1.

TABLE 1 Solid polymer electrolyte membrane Unit cell of fuel cellAntimony oxide particles (Sb₂O₅•nH₂O) Cell voltage Organic resin Averageparticle diameter n Content 80° C. 100° C. 120° C. 140° C. membrane (nm)value (wt %) (V) (V) (V) (V) Ex. 1 Nafion membrane (A) 10 2.5 20 0.630.64 0.61 0.58 Ex. 2 Nafion membrane (A) 10 2.5 35 0.64 0.66 0.63 0.6Ex. 3 Nafion membrane (A) 10 2.5 45 0.65 0.67 0.64 0.61 Ex. 4 Nafionmembrane (A) 40 2.5 15 0.62 0.63 0.59 0.55 Ex. 5 Nafion membrane (B) 102.5 25 0.63 0.63 0.60 0.56 Ex. 6 Nafion membrane (C) 10 2.5 15 0.62 0.610.58 0.53 Comp. Ex. 1 Nafion membrane (A) — — — 0.60 0.57 0.50 0.41Comp. Ex. 2 Nafion membrane (B) — — — 0.59 0.55 0.54 0.38 Comp. Ex. 3Nafion membrane (C) — — — 0.58 0.54 0.45 0.34

Example 7

Antimony pentoxide particles (average particle diameter: 10 nm,Sb₂O₅.2.5H₂O) as inorganic oxide particles having proton conductivitywere dispersed in a mixed solvent (ethyl alcohol:water=50:50) to preparea dispersion having a Sb₂O₅ concentration of 40% by weight. To thisdispersion, Ceramate 503 (available from Catalysts & ChemicalsIndustries Co., Ltd., SiO₂ concentration: 16% by weight) obtained byhydrolysis of methyltrimethoxysilane was added as a precursor of amatrix component in such a manner that the Sb₂O₅:SiO₂ ratio became50:50, and they were stirred at 50° C. for 1 hour to give a coatingmaterial (A) for forming inorganic electrolyte membrane having a totaloxide concentration of 20.8% by weight.

The coating material (A) was filled in a Teflon (R) mold of 10 cm×10 cm,heated up to 250° C. at a rate of 1° C./min and maintained at 250° C.for 6 hours to give an inorganic electrolyte membrane (A). The thicknessof the inorganic electrolyte membrane (A) taken out of the mold was 0.8mm.

Separately, to platinum-supported carbon particles having a platinumcontent of 40% by weight in terms of Pt, a mixed solvent of ethylalcohol and water (ethyl alcohol:water=50:50) was added to give a paste.Then, the paste was applied onto two sheets of carbon paper (availablefrom Toray Industries, Inc.), which had been subjected to waterrepellent treatment with tetrafluoroethylene, in such a manner that thedensity of the platinum-supported carbon particles on each sheet became0.5 g/cm², and they were dried at 100° C. for 12 hours to prepare twogas diffusion electrodes (B).

The two gas diffusion electrodes (B) were used as a positive electrodeand a negative electrode. Between these electrodes, the inorganicelectrolyte membrane (A) was sandwiched, and they were hot pressed at300° C. for 4 minutes under a pressure of 20 kg/cm² to prepare a unitcell (J) in which the gas diffusion electrodes (B) and the inorganicelectrolyte membrane (A) were joined.

(Evaluation)

The unit cell (J) was subjected to moistening treatment at 80° C. and arelative humidity of 30% for 2 hours. Then, the unit cell (J) wasoperated at a current density of 0.5 A/cm² for 50 hours at atmosphericpressure and a temperature of 80° C., 100° C., 140° C. or 180° C., andan output voltage at each temperature was measured. The results are setforth in Table 2.

Further, in order to evaluate high-temperature durability, the unit cell(J) was operated at 140° C. for 500 hours, and an output voltage wasmeasured. The result is set forth in Table 2.

Example 8

Antimony pentoxide particles (average particle diameter: 10 nm,Sb₂O₅.2.5H₂O) as inorganic oxide particles having proton conductivitywere dispersed in a mixed solvent (ethyl alcohol:water=50:50) to preparea dispersion having a Sb₂O₅ concentration of 30% by weight. To thisdispersion, Ceramate 503 (available from Catalysts & ChemicalsIndustries Co., Ltd., SiO₂ concentration: 16% by weight) obtained byhydrolysis of methyltrimethoxysilane and a zirconia sol (available fromDaiichi Kigenso Kagaku Kogyo Co., Ltd., average particle diameter: 5 nm,ZrO₂ concentration: 25% by weight) were added as precursors of matrixcomponents in such a manner that the Sb₂O₅:SiO₂:ZrO₂ ratio became50:40:10, and they were stirred at 50° C. for 1 hour to give a coatingmaterial (B) for forming inorganic electrolyte membrane having a totaloxide concentration of 21.9% by weight.

Then, a unit cell (K) was prepared in the same manner as in Example 7,except that the coating material (B) was used.

(Evaluation)

Using the unit cell (K), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

Example 9

Antimony pentoxide particles (average particle diameter: 10 nm,Sb₂O₅.2.5H₂O) as inorganic oxide particles having proton conductivitywere dispersed in a mixed solvent (ethyl alcohol:water=50:50) to preparea dispersion having a Sb₂O₅ concentration of 30% by weight. To thisdispersion, Ceramate 503 (available from Catalysts & ChemicalsIndustries Co., Ltd., SiO₂ concentration: 16% by weight) obtained byhydrolysis of methyltrimethoxysilane and a silica sol (available fromCatalysts & Chemicals Industries Co., Ltd., SI-350, average particlediameter: 7 nm, SiO₂ concentration: 20% by weight) were added asprecursors of matrix components in such a manner that theSb₂O₅:SiO₂:SiO₂ sol ratio became 70:20:10, and they were stirred at 50°C. for 1 hour to give a coating material (C) for forming inorganicelectrolyte membrane having a total oxide concentration of 24.5% byweight.

Then, a unit cell (L) was prepared in the same manner as in Example 7,except that the coating material (C) was used.

(Evaluation)

Using the unit cell (L), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

Example 10

A unit cell (M) was prepared in the same manner as in Example 7, exceptthat antimony pentoxide particles (Sb₂O₅.2.5H₂O) having an averageparticle diameter of 40 nm were used as inorganic oxide particles havingproton conductivity.

(Evaluation)

Using the unit cell (M), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

Example 11

Two gas diffusion electrodes (B) were prepared in the same manner as inExample 7. With respect to one of the gas diffusion electrodes (B), thecoating material (A) for forming inorganic electrolyte membrane preparedin Example 7 was applied onto a surface coated with platinum-supportedcarbon by a roll coating method, then dried at 100° C. for 12 hours andheated at 250° C. for 6 hours to form an inorganic electrolyte membrane(N) on the gas diffusion electrode (B). The thickness of the inorganicelectrolyte membrane (N) was 0.2 nm.

Thereafter, on the inorganic electrolyte membrane (N), the other gasdiffusion electrode (B) was placed, and they were hot pressed at 300° C.for 4 minutes under a pressure of 20 kg/cm² to prepare a unit cell (N)in which the gas diffusion electrodes (B) and the inorganic electrolytemembrane (N) were joined.

(Evaluation)

Using the unit cell (N), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

Example 12

Antimony pentoxide particles (average particle diameter: 10 nm,Sb₂O₅.2.5H₂O) as inorganic oxide particles having proton conductivitywere dispersed in a mixed solvent (ethyl alcohol:water=50:50) to preparea dispersion having a Sb₂O₅ concentration of 30% by weight. To thisdispersion, Ceramate 503 (available from Catalysts & ChemicalsIndustries Co., Ltd., SiO₂ concentration: 16% by weight) obtained byhydrolysis of methyltrimethoxysilane and a dispersion of rare earthion-exchanged zeolite (REY, available from Catalysts & ChemicalsIndustries Co., Ltd., SiO₂/Al₂O₃=8, average particle diameter: 0.8 μm),which has a SiO₂.Al₂O₃ concentration of 30% by weight, were added asprecursors of matrix components in such a manner that theSb₂O₅:SiO₂:(SiO₂.Al₂O₃) ratio became 50:30:20, and they were stirred at50° C. for 1 hour to prepare a coating material (O) for forminginorganic electrolyte membrane having a total oxide concentration of23.7% by weight.

Then, a unit cell (O) was prepared in the same manner as in Example 7,except that the coating material (O) was used.

(Evaluation)

Using the unit cell (O), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

Comparative Example 4

Two gas diffusion electrodes (B) were prepared in the same manner as inExample 7. The two gas diffusion electrodes (B) were used as a positiveelectrode and a negative electrode. Between these electrodes, aperfluorocarbon sulfonic acid membrane (A) (available from DuPont,Nafion membrane N-117, thickness: 183 μm) was sandwiched, and they werehot pressed at 100° C. for 5 minutes under a pressure of 150 kg/cm² toprepare a unit cell (P) in which the gas diffusion electrodes (B) andthe perfluorocarbonsulfonic acid membrane (A) were joined.

(Evaluation)

Using the unit cell (P), output voltages were measured in the samemanner as in Example 7. The results are set forth in Table 2.

TABLE 2 Inorganic electrolyte membrane Antimony oxide particles AverageWater retentive Unit cell of fuel cell particle Matrix particles Cellvoltage Durability diameter n Content Content Content 80° C. 100° C.140° C. 180° C. 140° C./500 hr (nm) value (wt %) (wt %) (wt %) (V) (V)(V) (V) (V) Ex. 7 10 2.5 50 SiO₂ 50 — — 0.63 0.64 0.61 0.56 0.59 Ex. 810 2.5 50 SiO₂ 40 — — 0.65 0.66 0.63 0.58 0.61 ZrO₂ sol 10 — — Ex. 9 102.5 70 SiO₂ 20 — — 0.66 0.67 0.64 0.59 0.63 SiO₂ sol 10 — — Ex. 10 402.5 50 SiO₂ 50 — — 0.63 0.63 0.59 0.53 0.57 Ex. 11 10 2.5 50 SiO₂ 50 — —0.64 0.65 0.62 0.57 0.6 Ex. 12 10 2.5 50 SiO₂ 30 zeolite 20 0.62 0.610.57 0.51 0.53 Comp. — — — — — — — 0.61 0.57 0.26 0 0 Ex. 4

INDUSTRIAL APPLICABILITY

According to the present invention, the electrolyte membrane comprisesan organic resin or an inorganic matrix component, and inorganic protonconductive oxide particles having excellent water retention propertiesat high temperature and conductivity, and therefore, a fuel cell capableof stably maintaining a high output voltage even in case of a prolongedoperation and an operation at a high temperature of 100° C. or highercan be provided. Especially when an inorganic matrix component is used,a fuel cell having enhanced, excellent stability at high temperature andexhibiting high cell performance even in a prolonged operation and anoperation at high temperature can be provided.

1. An electrolyte membrane comprising inorganic oxide particles havingproton conductivity and a matrix component, wherein the inorganic oxideparticles having proton conductivity comprise hydrated antimony oxideparticles represented by the following formula (1) and have an averageparticle diameter of 5 to 50 nm, and the content of the hydratedantimony oxide particles is in the range of 5 to 75% by weight in termsof an oxide (Sb₂O₅);Sb₂O₅.nH₂O  (1) wherein n is 0.1 to
 5. 2. The electrolyte membrane asclaimed in claim 1, wherein the matrix component comprises an organicresin.
 3. The electrolyte membrane as claimed in claim 2, wherein theorganic resin is at least one organic resin selected from the groupconsisting of a cation-exchange resin based on polystyrene, a mixture offluorocarbon sulfonic acid and polyvinylidene fluoride, a graftcopolymer wherein trifluoroethylene is grafted on a fluorocarbon matrix,a perfluorocarbon sulfonic acid resin, a vinylidene fluoride resin, a2-dichloroethylene resin, a polyethylene resin, a vinyl chloride resin,an ABS resin, an AS resin, a polycarbonate resin, a polyamide resin, apolyimide resin and a methacrylic resin.
 4. The electrolyte membrane asclaimed in claim 1, wherein the matrix component is an inorganic matrixcomponent.
 5. The electrolyte membrane as claimed in claim 4, whereinthe inorganic matrix component comprises at least one inorganic oxideselected from the group consisting of ZrO₂, SiO₂, TiO₂ and Al₂O₃.
 6. Afuel cell using the electrolyte membrane of claim
 1. 7. A fuel cellusing the electrolyte membrane of claim
 2. 8. A fuel cell using theelectrolyte membrane of claim
 3. 9. A fuel cell using the electrolytemembrane of claim
 4. 10. A fuel cell using the electrolyte membrane ofclaim
 5. 11. The electrolyte membrane as claimed in claim 3, wherein theorganic resin is a perfluorcarbon sulfonic acid resin.